The present invention relates to a magnetic recording and reproducing apparatus for a video signal and, particularly, to a recording inhibition function to be performed thereby in response to a recording inhibition signal attached to an input video signal.
In order to protect information recorded on a commercially available magnetic tape such as soft tape, it is necessary to make an unauthorized dubbing of it onto another magnetic tape impossible. An example of a device for making such protection possible is disclosed in Japanese Kokai (P) No. 61-288582.
In the disclosed device, a recording inhibition signal called a "copy guard" signal is attached to a video signal recorded on a soft tape during a vertical blanking period thereof. When the video signal is reproduced and supplied to another magnetic recording and reproducing apparatus, an automatic gain control circuit provided in a recording processing system of the magnetic recording and reproducing apparatus for optimizing a level of the video signal for recording may malfunction due to the recording inhibition signal make the level of the video signal to very low so that it can not be recorded.
The recording inhibition signal comprises a plurality of paired pseudo synchronizing signals and positive pulses and is added between adjacent equalizing pulses in a vertical retrace period or between horizontal synchronizing signals. The pseudo synchronizing signal is in phase with the synchronizing signal and the positive pulse is out of phase with the synchronizing signal and has a level not lower than white level. The automatic gain control circuit provided in the recording processing system of the magnetic recording and reproducing apparatus usually detects the synchronizing signal of the video signal, detects the back porch level using the detected synchronizing signal as a timing reference and, in response to the detected back porch level, controls gain to make the level of the video signal optimum for recordation.
When the video signal to which the recording inhibition signal is attached is supplied to the automatic gain control circuit, the pseudo synchronizing signal is detected thereby in a similar manner to the synchronizing signal and a level of the subsequent positive pulse is detected using the pseudo synchronizing signal as a timing reference. Since, however, the level of the positive pulse is white level or higher which is very high compared with the back porch level, the gain of the automatic gain control circuit becomes very low, so that the video signal becomes too low to be recorded. Thus, the video signal can not be recorded or reproduced satisfactorily.
In general, in one operation mode of a magnetic recording and reproducing apparatus, there is a case where an external video signal supplied to the magnetic recording and reproducing apparatus is not recorded thereby but supplied thereby to a television receiver for monitoring thereon. Since, in such case, the video signal supplied to the television receiver is outputted from the automatic gain control circuit of the magnetic recording and reproducing apparatus, a level of the signal is also optimized.
In such mode of operation, if the external video signal is provided with such recording inhibition signal, the gain of the automatic gain control circuit may be set to a very low value, causing the video signal level to be very low and thus monitoring to be impossible.